WASHINGTON (AP) — There's growing evidence that most of the dramatic gain in the number of Americans with health care coverage is due to President Barack Obama's law, and not the gradual recovery of the nation's economy.

That could pose a political risk for Republicans running against "Obamacare" in the GOP primaries as they shift to the general election later this year. While the health care law remains highly unpopular in the party, the prospect of taking away health care coverage from millions of people could trigger a backlash if the eventual GOP nominee's plan to replace it is seen as coming up short.

"There are different phases of the campaign," said GOP pollster Bill McInturff. Playing to like-minded voters in the primaries, Republican front-runner Donald Trump doesn't have to spell how he'd replace Obama's law. "When you get to the general election, the demand for what you are going to do different starts to escalate."

Under "Obamacare," the share of Americans without health insurance has dropped to a historic low of about 9 percent, with room to go even lower. But even as the economy has expanded, major government surveys point to a lackluster rebound for employer-based coverage.

"It's very clear that the Affordable Care Act has done most of the work in decreasing the number of uninsured," said economist Robert Kaestner of the University of Illinois at Chicago.

The numbers vary across different government surveys, but the overall pattern is strikingly similar:

— The Census Bureau's American Community Survey found about 3 million more people gained employer coverage between 2010, when the health law passed, and 2014. But the number of uninsured people dropped by more than 10 million during that same period. The strongest gains appeared to come from Medicaid, which was expanded under Obama's law. The percentage of Americans covered by employers stayed about the same.

— The Census Bureau's Current Population Survey found no statistically significant change in the number of people covered by employers from 2013 to 2014, while the number of uninsured people dropped by nearly 9 million as the law's main coverage expansion got under way during that time.

"This kind of shift in insurance I don't think can be explained by the economy," economist Christine Eibner of the RAND Corporation said. "The increase (in coverage) is large enough that it can't be driven by just economic recovery."

Kaestner said "most of the heavy lifting" seems to be coming from Medicaid expansion.

Employer-provided insurance plans remain the mainstay for workers and their families, covering an estimated 150 million to 170 million Americans. But even before the 2007-2009 economic recession, workplace coverage was steadily shrinking because of rising medical costs.

Obama's law provides subsidized private insurance for those who don't have access at work, along with a Medicaid expansion geared to low-income adults in states that agreed to do so. Most individuals are required to have coverage, and larger employers must offer it or face fines.

The health care law has been difficult to navigate for consumers, and its skinny policies can expose patients to high medical bills. But it's becoming a backstop for millions of Americans in a changing economy.

On the campaign trail, Republican presidential candidates denounce "Obamacare" for a litany of woes. But some prominent conservative experts recognize that the law has increased coverage, even as they propose other approaches to meet that goal.

"Repealing the law without a plausible plan for replacing it would be a mistake," said a policy paper from 10 leading GOP health policy experts, published by the business-oriented American Enterprise Institute.

Democratic front-runner Hillary Clinton has already previewed how Democrats might use the issue this fall, frequently reminding voters they risk losing some popular benefits if the health care law is eliminated. Meanwhile, a nonpartisan analysis of Trump's initial outline for repealing and replacing the health care law found it would push millions back into the "uninsured" category.

The analysis last week from the nonpartisan Committee for a Responsible Federal Budget found that the Trump plan would increase the number of uninsured by about 21 million people while costing the government nearly $500 billion over 10 years.

Replacing Obama's law with a conservative alternative that delivers comparable coverage would require considerable taxpayer dollars, something few Republicans seem ready to accept.

"Any repeal has to have a way to increase coverage and not just by a few million," said economist Gail Wilensky, who ran Medicare under former President George H.W. Bush.

Obama's law "is obviously not the only way to do this, but it is important that it gets done" Wilensky added.